Process of coating base metals with aluminum



Aug. 7, 1962 G. A. MLLl-:R

PROCESS 0F coAIING BASE METALS WITH ALUMINUM 6 Sheets-Sheet 1 Filed Feb.

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INV ENTOR.

llg- 7, 1962 G. A. MLLER 3,048,497

PRocEss oF coATING BASE METALS WITH ALUMINUM Filed Feb. 1s, 1958 esheets-sheet 2 aus@ BMM f@ vin/orne .s

IN VEN TOR. len/v ,que 0.57- /W Aug. 7, 1962 G. A. MLLER 3,048,497

PROCESS 0F comme BASE METALS WITH ALUMINUM Filed Feb. 1a, 195e 6Sheets-Sheet 3 INVENTOR. Gamm Hususr MOL/.EQ

Aug. 7, 1962 G. A. MLLER PROCESS 0F COATING BASE METALS WITH ALUMINUM 6Sheets-Sheet 4 Filed Feb. 18, 1958 K ALUM/NUM Aug. 7, 1962 G. A. MLLERPRocEss oF coATING BASE METALS wITM ALUMINUM Filed Feb. 18, 1958 6Sheets-Sheet 5 wuzuz. A wwzuz. whc SS Qu 0 O COATDNG POTENTIAL, VOLTSAIQ COOLED INVENTOR.

Go'femv ,Qual/57M OLLEB 3,048,497 PROCESS 0F COMING BASE METALS WITHALUMINUM G. A. MLLER 6 Sheets-Sheet 6 Aug. 7,1962

Filed Feb.

INVENTOR. 60e/cw ususrMoLLf/a United States Patent Oiilce igil PatentedAng. 7, i952 3,048,497 PROCESS F CATHNG BASE METALS WHH ALUMINUM GranAugust Mller, Gotene, Sweden Consuiate General of the United States ofAmerica, Cape Town,

Union of South Africa) rues nee. is, 195s, ser. No. 716,257 21 Claims.(Cl. 117-52) The present invention relates generally to the art of metalfinishing, and more particularly, to a method of pre-treating metalobjects to be coated and thereafter coating them by immersion in amolten metal bath.

Metal coating processes such as galvanizing, i.e., coating with zinc,are well known in the art. Heretofore, however, such coating processeshave been confined to relatively few coating metals, among them zinc,which will form a continuous homogeneous protective layer on the metalto be protected and which will wet, that is, adhere rrnly to the parentmetal so as not to be removed by subsequent rough handling of the coatedarticles, and so as not to leave voids, pits, pinholes, and otherblemishes in the coating.A It is also axiomatic, of course, that thecoating metal be one which has a relatively high resistance tocorrosion, wear and the like. Still further, it is desirable that thecoating metal be relatively inexpensive.

Aluminum is admirably suited'to the function of a coating metal from thestandpoint of its resistance to corrosion and its relatively low cost.Aluminum has the property of forming an loxide layer on its own exposedsurface which layer is thereafter highly resistant to further corrosionby the elements and is also quite resistant to abrasion and the like.Aluminum has the further advantage of being highly malleable so thatbending or other distortion of the parent metal having an aluminumcoating thereon does not tend to crack the coating.

In coating metals such as iron, copper, titanium and nickel withaluminum by hot-dip methods such as disclosed in my prior patent, No.2,315,725, entitled Process for Metallization, especially Aluminizationof Iron Articles, issued April 6, 1943, a separate layer of base metaland aluminum forms at the interface between the outer aluminum coatingand the base metal surface. This interfacial layer formation acts as abonding mediumV between the base metal and the aluminum. It isimportant, however, that this interfacial base metal-aluminum layer bekept as thin as possible inasmuch as it is found that the ductility ofthe layer increases as the thickness of the interfactial layerdecreases. Hereinafter the term layer is referred to as an alloy oralloy layer inasmuch as it appears that the interface formed between thebase metal and aluminum is a mixture of these metals. The term aluminumjas used herein, includes the alloys of aluminum.

In addition to the importance of minimizing the alloy layer, it ishighly desirable to produce, in combination therewith, a maximumthickness of an aluminum coating.

The substantial removal-of all the oxides and other contaminants fromviron and other base metal surfaces just prior to the coating step isessential for adequate formation of the bonding interfacial alloy layer.Hot-dip aluminizing processes, embodying a molten salt pretreatment stepjust prior to coating for the removal of oxides, as well as otherfeatures are disclosed in my prior aboveidentified patent.

The present apparatus and process constitutes in general an improvementover that shown in the aforesaid patent. This 'application is acontinuation-impart of my copending application, Serial No. 252,267,entitled Method and Means for Cleaning and Coating Metal Objects, nowabandoned.

Bearing in mind the foregoing facts, it is a major object of the presentinvention to provide an improved means and a method for coating metalobjects by immersing the same in a different molten metal.

Another object of the invention is to provide improved apparatus andmethods in which the coating metal is selected from the group consistingof aluminum and aluminum alloys;

it is also an object of the invention to provide a method and means forpretreating, by chemical and electrolytic action, metal objects prior toand in combination with an immersion-coating process and apparatus.

A further object of the invention is to provide a method and means forelectrolytically pickling base metal objects in combination with amethod and means for immediately thereafter immersion-coating said basemetal objects without intervening oxidation, whereby the thickness ofthe resulting interfacial alloy layer is substantially decreased whileat the same time substantially increasing the thickness of the aluminumcoating.

It is still another object of the invention to provide a method andmeans for aluminizing a base metal object which includes the step ofemploying said base object as an electrode during the actualaluminization step with the molten salt as the other electrode, thedirection of current flow between the object and the salt bath producinga maximum aluminum coating with minimum interfacial alloy formation forany given voltage application and for any given type of quenching.

Another object of the invention is to provide a method and means foraluminizing a base metal object by first preparing said object foraluminizing by electrolytic pickling in a molten salt bath, said objectacting as one electrode and said salt bath as the other electrode, by

second employing said object as an electrode during the actualaluminization step with the molten salt as the other electrode, thedirection and duration of current flow between the object and the saltbath being such as to maximize the thickness of the coating and minimizethe formation of the alloy layer for any givenvoltage application, andby third air-cooling the metal object after coating.

Yet another object of the invention is to provide apparatus by which anelectric current may be passed between the base metal and the coatingmetal during the coating process and between the base metal and cleaningbath during the pretreating process.

A further object of the invention is to provide apparatus by which theimmersion-coating of certain base metal objects or work pieces iscarried on concurrently with the pretreating process of other workpieces, and in which the same electric current flows vthrough all workpieces involved in such concurrent operation.

Still another object of the invention is to provide apparatus of theclass described having a novel means for mechanically and electricallyconnecting work pieces to a work-supporting fixture.

The foregoing objects and advantages of the invention will be apparentfrom a consideration of the following description of preferred formsthereof and from the attached drawings, in which a' `FIGURE l is apartially sectioned perspective view of an electrically heated salt bathfurnace constructed and adapted to carry out one form of the presentinvention;

FIGURE 2 is an elevational section taken on the line 2 2, in FIGURE 1;

FIGURE 3 is a plan view of the furnace illustrated in FIGURES 1 and 2;

FGURE 4 is an enlarged fragmentary elevational section showing a workpiece during the immersion-coating phase of a modified form of myprocess.

FIGURE 5 is a partially sectioned perspective View of `a modified saltbath furnace showing an alternate form of the invention;

FIGURE 6 is an enlarged fragmentary elevational section taken on theline 6-6 in FIGURE 5 and showing a work-supporting fixture with workpieces thereon;

FGURE 7 is a plan view of the apparatus shown in FEGURE including thework-supporting fixture shown in FIGURE 6;

FIGURE 8 is an enlarged fragmentary elevational seetion of a modifiedwork-supporting fixture showing an alternate modified form of thework-supporting fixture shown in FIGURE 6;

FlGURE 9 is a transverse elevational section taken on the line 9 9 inFIGURE 7, showing a `further modification of the invention;

FIGURE 10 is a similar transverse elevational section showing a stillfurther modification;

FiGURES lla and 11b are photomicrographs of aluminized steel Ydepictingthe aluminum coating thickness, and the alloy layer thickness with andwithout the use of current;

FIGURE 12 is a graph showing the eect of the application of variouspotentials on aluminum thickness and interfacial alloy thickness whenthe specimen is air cooled after coating; and

FIGURE 13 is an elevational section of a modified form of salt bathfurnace.

In general, the present invention includes many of the yfeatures shownin my above-mentioned prior patent. There, a salt `bath furnace isdisclosed in which a layer of molten aluminum is floated on a bath offused or molten salt and in which the articles to be coated are held inthe salt bath until they reach the temperature thereof, and until thepickling action of the molten salt removes impurties from the surface topresent a chemically clean surface to be coated by the aluminum. Afterreaching the desired temperature as just described, and after beingcleaned, the work piece is then withdrawn from the salt through themolten aluminum layer, such passage through the molten aluminum causinga coating of the molten metal to adhere to the parent metal.

In the process described in Mller Patent No. 2,315,725, a substantialaluminum-iron interface is 0btained between the surface of the metal andthe aluminum coating. It is true that some ferro-aluminum alloy betweenthe parent metal and aluminum coating is helpful since the alloyadheresrmly to both iron and aluminum, thus forming a bond between theparent metal and the coating. The present invention, however, provides aprocess of aluminization whereby amore uniform and thinner, andtherefore more ductile aluminum-iron interface is obtained withoutsacrificing the thickness or quality of aluminum coating and indeed,increasing the thickness thereof.

The present invention utilizes a fused bath compri-sing a mixture ofsalts. The molten aluminum layer rests upon this salt mixture. Oneparticularly advantageous salt mixture has been found to `be thatcomprising approximately 65-85% lbarium chloride, and 15-35% sodiumchloride, the latter being added to reduce the fusing temperature of theresulting mixture. Such a salt mixture 'has a specific gravity ofapproximately 3 to 4, whereas aluminum has a specific gravity ofapproximately 2.7.I

The fusing temperature of the just-described salt mixture is in theneighborhood of 1200 F., and the melting temperature of aluminum isapproximately 1280" F. Thus, when the salt is fused and raised above itsfusing temperature to a temperature between 1200o and 1300 F., thealuminum is also maintained in a just-molten state.

Other salts which are useful in the processes herein described are ingeneral the halides of barium, potassium, sodium, and aluminum. Forexample, sodium aluminum fluoride is useful in dissolving oxides from aWork piece to be coated.

For a more detailed description, reference should be d had now to FIGURE1 of the drawings, wherein a salt bath furnace is identified generallyby the reference character 1d, being provided with heating electrodes 11which operate in the known manner to pass current through the moltensalt to maintain the same in its molten condition. In the presentinstance, alternating heating current is employed, thus giving rise to acirculation of the molten salt produced by the eddy currents flowingtherein.

It will be understood that the means employed to heat the molten salt inthe furnace 10 do not form an essential part of the present invention,and thus no detailed description thereof appears herein. It will also berealized that other heating means, such as gas burners, coal burners,oil burners, and the like could be employed to heat the furnace 1t?without altering the characteristics of the invention described herein.

A lower process (as distinguished from heating) electrode f5 isconstructed in a U-shape to extend around three sides of the furnace andis supported on a conductor bar 16 near the bottom of the furnace 10,the electrode 1.5 being of metal, for example, iron. The processelectrode 15 rnay be completely closed, eg., 4-sided, if desired.

As can also be seen from the phantom line liquid level shown in FIGURE2, a molten layer of aluminum 20 is 'floated on top of the molten salt21, a sleeve 19 of ceramic or other insulating material being placedaround the upper portion of the conductor bar f6 so as to prevent itscontact with the molten aluminum 20.

When it is desired to pretreat a work piece 23, the same is suspended onthe hook-shaped lower end of a vertical metal work-supporting rod 23,substantially completely enclosed by a ceramic sleeve 19a or othersuitable insulating sleeve and placed near the bottom of the furnace 1@in close proximity to the electrode 15. The current source is connecteddirectly to the metal rod 29, as shown schematically in FIGURES 1 and 2,so as to make the metal rod 29 positive with respect to the lowerelectrode f5. Inasmuch as the work-suspending fixture 29, iselectrically insulated from the molten aluminum 20, current flowsdownwardly through the fixture 29 and into the work piece 23, making thelatter positive (anodic) with respect to the lower electrode 15(cathodic).

The time of immersion of piece 28 varies depending upon shape,thickness, material and end use of the piece 23 (whether it should beheat-treated or not), the geometry of the system, aswell as otherfactors. The period of immersion can thus be generally varied betweenseveral seconds to 10 minutes.

During this period `of time, the work piece 23 is thoroughly cleansedand the oxides removed electrolytically. As will be described in moredetail hereinafter, it has been found that greater pick-ling voltages(i.e., the Work 28 always being positive with respect to the salt bath21) during the immersion of the work piece 23 or soaking results information of thicker aluminum layers upon the work piece. For example,it has been found that as the EMF. applied during the soaking period isincreased from 0-6 volts, (all other conditions being equal) thethickness of the aluminum coating subsequently produced increasedlbetween 3 and 4 times. This phenomenon is believed to be due to anincrease in roughness of the surface of the parent metal withAincreasing soaking voltage, `aluminum having greater yadhesion to theroughened surface. On the other hand, the iron-aluminum alloy layersubsequently produced (all other conditions being equal) increased inthickness but in minor amounts in comparison as the was increased from0-6 volts. In most instances, the workp iece 2S remains positive as itis drawn through the coating aluminum layer 20. The coated steel is thenquenched in an oil or other low temperature bath (not shown) which issufficientlyA large with respect to the coated steel to be retained at500 F. or lower throughout the quench. This process is found to be muchsuperior to a process employing the same aluminum-molten salt system,`as shown in FIGURES l and 2, but without any direct current input.

Air, or other relatively slow cooling steps, may also be employedresulting in austenitic and pearlitic steel formations if steel is usedas the base metal. However, the interfacial steel-aluminum alloy therebyproduced has a much greater chance of formation and is in fact,substantially thicker than in oil quenching. A corresponding decrease inthe aluminum coating thickness also results. Oil cooling in addition topreventing large interfacial alloy formation, also may bring about theformation of hard martensitic steel if the quench begins at a hightemperature, i.e. above its critical of 1333" F., and is quenchedrapidly enough i.e. above the critical cooling rate. Thus, the steel maybe coated at above l333 F. and quenched below about 550 F. therebyforming 4a substantially martensitic steel core.

Air cooling of the coated metal is nevertheless sometimes used,depending principally upon the type of base metal employed and the enduse desired. Even with aircooling, it is still highly desirable toproduce the smallest interfacial layer thickness With the largestaluminum coating thickness, and a substantial modication of the basicdirect current process just described is employed in conjunction withthe air-cooling step to achieve this result.

This modification best shown in FIG. 4 requires that, just as the Workpiece 28 enters the aluminum layer, the of the system be reversed. Thusthe Work piece 23 becomes cathodic with respect to the molten salt bathduring, but not prior to, the coating period. The timing of the reversalof the E.M.F., as just described, is critical in the obtaining ofoptimum results. Thus, if the voltage is reversed even several secondsprior to the commencement of `the coating period, it is `found that thealuminum does not adhere as Well to the work piece 28. This phenomena isbelieved to be due to the electrodeposition of metallic sodium or bariumon the prematurely negative Work piece 28, which then prevents goodadherence of the aluminum unless these metals are dissolved or wiped offin the aluminum float 20.

Referring specifically to FIGURES lla and 1lb, these depict actualphotomicrographs of pieces of aircooled aluminum coated steel producedwith and Without the use of direct current respectively. The pertinentdata for each photomicrograph is listed below:

FIGURE 11a Magnification 1500. v Size of Wire ..080". j Grade of steelEutectoid composition.

Temperature of Al bath l430 F. Speed of rWire thru bath 60 fpm.

Volts l0 v. D C.

Amperage 2.00 amp.

Remarks Positive pole in Al. layer 20. Cooling ...In ambient air.

Thickness of coating .Outer layer (Al.) 00072 av. Inner layer (Al.alloy) .0007.8 av., Total `layer 00100 av.

FIG URE 11b Magnification 1500. Size of Wire .080". Grade of steelEutectoid composition. Temperature of Al bath l430 F. Speed of vwirethru bath 60 fpm. Etching solution '1/2 HFT-2% Nital. Remarks Runwithout direct current. Cooling In ambient air. Thickness of coatingOuter layer (Al.) .00038" av., Inner layer (Al. alloy 00039 av., Totallayer 00100 av.

The percentage of aluminum of the total layer formed by applying a-ivoltage to the aluminum layer during both soaking and coating periodsis 72.0% and is substantially larger than the coating of the same gradeof steelwithout the use of current (43.9%). Correspondingly, the totalthickness of `the interfacial layer is 50.7% of the total layer withoutuse of current and but 28.0% with the use of current.

Specifically referring now to FTGURE l2, the results of actual testsshowing the mean thickness of aluminum deposition and the mean`thickness of steel-aluminum alloy formation respectively on theair-cooled specimens, for various soaking and coating voltages, arepresented. The process `followed in obtaining the data for FIGURE l2included the reversal of EMF. as just described. The mean thickness wasdetermined by photomicrographic survey of `air-cooled insulated stelspecimens to which the voltage Was directly applied. The term negativecoating potential means that the work 23 is negative With respect to thesalt bath 2l. Conversely, a positive soaking or coating potential meansthat the Work 23 is positive with `respect to the salt bath 2l duringthe soaking or coating phases respectively.

`In general, FIGURE l2 shows that `a substantially thicker aluminumcoating is produced with increasing negative coating voltages.Conversely, the iron-aluminum intermetallic layer becomes thinner as thenegative coating voltage increases. If positive coating voltages wereemployed instead of negative voltages, it can be seen that some decreasein percentage of aluminum results. For example, at +6 volts soaking, andat +6 volts coating, the thicknesses of the various layers is as`follows:

Aluminum, 0.0038 inch-% aluminum of total layer. Aluminum-steel alloy,0.0016 inch-30% iron-aluminum alloy of total layer.

On the other hand, at +6 volts soaking, `and -6 volts coating, thethickness of the layers is:

Aluminum, 0.0047 inch-84% aluminum of total layer. Aluminum-steel alloy,0.0009 inch- 16% iron-aluminum alloy of total layer.

Thus, for this particular set of voltage conditions set forth, theactual increase of aluminum deposited is 0.0009 of aluminum, `or

0.0009 in. m 24 Increase Further, the actual decrease in aluminum-steelalloy is 0.0007 inch of alloy or 43 decrease over the amount present inpositive soaking and coating phases.

As mentioned, it is found that a thinner intermetallic layer reduces thepossibility of rupture of the aluminum layer during bending and formingoperations, and the production of air-cooled aluminum coated metals bymy method therefore greatly enhances their ductility and formingcharacteristics.

By way of further contrast, it should he noted that at 0v soaking and 0Vcoating (that is Without the use of direct current in either the soakingor the coating phases of the process) the aluminum thickness is only0.0017 inch, and the steel-aluminum thickness is 0.0009 inch. Thus, thepercentage of aluminum of the total Ilayer is approximately only 35%,very substantially below the absolute amounts and percentages ofaluminum deposited obtained with the use of current. The steel-aluminumthicknesses are also very substantial.

Theoretically, it appears that the reason for the advantageous effectsof current reversal is that an extremely spaans? thin lm of sodiumand/or barium forms on the work piece which inhibits subsequentinterfacial alloy formation thereby decreasing the thickness of thislayer and correspondingly increasing the aluminum layer.

It can also be seen that increasing the posiitve applied to the workpiece 2S during the soaking period (at constant coating potential)results in a 20G-400% increase in the aluminum ylayer thickness, whilethe steelaluminum alloy layer either decreases or, if increasing,increases substantially less -rapidly in thickness. Thus, taking 3v asthe fixed coating potential, the percentage decrease in thealurninum-steel alloy in going from soaking to 6v soaking was On theother hand, the percentage of `aluminum increase was approximately ftical lmatter, the increased quality of oil cooled product,

due to the reversal, usually does not warrant the increased expenditureof providing for a current reversal ste ign `summary then, my oil cooledcoated steel product is generally produced without an reversal stepwhile my air cooled coated steel is generally manufactured with anreversal step. More specifically, one preferred process includes l) asoaking phase Wherein the work piece Z8 is made anodic and thenelectrolytically pickled until it reaches the approximate temperature ofthe salt bath 21, (2) a coating phase wherein the direction of currentis -reversed just as the work piece 28 is drawn into the overlyingaluminum layer 26 to be thereby coated. (3) an air cooling of the coatedWork piece 28.

Another preferred form of my process includes (l) a soaking and `coatingphase wherein the work piece 28 is made and maintained anodic withrespect to the negative salt bath (2) a severe cooling, such as in oilor water.

v Either of the preferred forms of my process has successfully employedthe following metals as base metals: titanium, vanadium, chromium,manganese, iron, cobalt, nickel, copper and alloys thereof. v

A modification of the apparatus of lFIGURES 1 to 3 has also been usedwherein the aluminum float 20 becomes the electrode by insertion of aconducting bar 17 as best shown in FIGURE 13. In this modification, themetal Work piece 28 is not insulated so that the current flows duringsoaking Afrom the aluminum 20 to the -metal support 29a and thence tothe work piece 28. [In this modification, While useful process-wise ineliminating the need for insulated work supports 29, the advantageouseffects of applied voltage are not as great as with the process referredto in FIGURES 1 to 4. It is thought that the large area of aluminum 20to salt 21 contact compared to that of Work piece 28 to salt contactpermits some shortcircuiting of the current otherwise flowing from thealuminum to the work piece. This short-circuiting phenomenon wouldreduce the magnitude of the current and the advantageous effects,above-described, due to it.

While inthe examples noted, the applied E.M.F. ranged 'from 6V to +6",the applied E.M.F. has been increased considerably where necessary toget sufficient current iiow in any particular system.

In FIGURES 5 through 7, a modified salt bath furnace 35 is provided witha central partition 36 which does not extend clear to the bottom of thefurnace 35, whereby to permit the substantially uniform heating of theentire bath due to the interconnection of the salt bath under the loweredge of the partition 36.

In this modified form of the invention, a shroud-like electrode 37 issuspended by a conductor bar 38 in the right-hand section 39 of thefurnace 35, whi-le upper electrode bars 40, similar to the conductingbar 17 of the embodiment described with reference to FIGURE l3 aresuspended in the left-hand furnace section 41. Heating electrodes 42,similar to those employed in the previous embodiment, are mounted in thetwo sections of the furnace 35, although it will Ibe realized that inthis embodiment as before, other means for heating the salt bath may beemployed.

As can be seen in FIGURE 6, an aluminum layer 43 is iioated on top ofthe salt 44 but only in the left-hand section 41 of the furnace 35.

A bifurcated work-supporting xture 50 having a Vertical shank S4 ispositioned to straddle the partition 36, one leg 51 of the xtureextending downwardly into the left-hand furnace section 41, and theother leg 52 extending into the right-hand section 39. Work pieces 28are suspended on hooks 53 extending laterally from both legs of theiixture Sti. The work pieces positioned in the righthand furnace section39 are in proximity to the shroud electrode 37 and are electrolyticallypickled in the molten salt, as previously described with reference toFIG- URE 4.

It will be realized that FIGURE 6 illustrates the process in media res,that is, the work pieces in the section 41 have already been pretreatedin the section 39.

'During the soaking phase, in the form of the device shown in FIGURES 5and 6, the current ilow is from the bar 40 to the aluminum layer 43, upthrough lthe transverse bar of the fixture 50, down the leg 52, and intothe Work pieces 28 and thence to the negative shroud electrode 37. Thedirection of the current between the aluminum 43 and the salt 44 may bereversed just prior to the immersion of a work piece 28 into the moltenaluminum 43 by the interconnection of appropriate independent DsC.circuits between the molten salt 43 and each of the work pieces (theD.C. circuits not being shown). The advantages of current reversal withair-cooling may be thus obtained in this modification, as well as theadvantages of direct current without reversal and with rapid cooling.

It should also be understood that the work supports of FIGURES 5 to 7can be insulated from the aluminum bath 43 and the current introducedalong the work support 50 itself, as described with reference to FIGURESl to 4.

In a further modification of the two-compartment furnace of FIGURES 5through 7, the partition 36 is extended clear to the bottom of thefurnace so as to completely separate the -two sections 39 and `41. Inthis way, different salts may be used in the different sections, that inthe coating section 41 being used solely for keeping the yaluminummolten and preheating the work piece.

As soon as one set of work pieces 28 has been pretreated and the otherset coated by drawing the same up through the molten aluminum layer 43,the latter set of Work pieces is removed from the hooks 53 and theiixture 50 rotated about the axis of the central supporting shank 54 soas to place the leg 52 in position to be immersed in the aluminum bathsection 41 of the furnace 35. Before lowering the ixture 50 to immersethe legs 51 and 52, a set of untreated work pieces 28 is placed on thehooks 53 of the leg 5I whereby to be immersed in the pre-treatingsection39 of the furnace 35.

Thus it will be seen-that the process performed by the apparatus asshown in FIGURES through 7 is practically continuous, and that the `mostefficient use of the processing current is employed since it is usedboth in the coating and the pretreatment phases of the process.

Still further it should be noted that in the embodiments of theinvention thus far discussed, such power as is absorbed by either themol-ten aluminum bath or by the salt bath is dissipated as heat therein,and thus is not wasted.

In some instances, such as `where the parts to 'be coated areconstructed of sheet metal, it is diiiicult to effect a secure,low-resistance connection between the work piece and the work-supportingfixture 29 or Si). This is particularly diiiicult when thework-supporting xture is at the elevated temperature produced byimmersion in the salt bath. To obviate this difficulty, the modication.illustrated in FIGURE 8 is provided.

Here it will be seen that a metal cylindrical container 60, having anopen upper end, is welded to the vertical shank of a work-supportingfixture 51a which may be either one leg of the fixture Sil or thevert-ical shank of the nxture 29 employed in the first embodiment. Inthe container o@ is placed a bath of lead 61 or other metal having a lowmelting temperature and a relatively high specic gravity as compared toeither the salt bath or the aluminum. Upon immersion in the salt bathfurnace, the lead 61 is melted and is thus adapted to receive, immersedtherein, an arm 62 of the clamping hook 63 which is securely fastened toa work piece 28 by suitable means such, for example, as a bolt 64 andwing nut 65. Alternatively, the hook connector 63 may be Hash-welded tothe work piece 28 and removed after the coating process is completed.

A substantial length of the hook arm 62. being immersed in the lead bath61, a low resistance electric connection is effected between the xturearm Sla and the work piece Z3. Also it will be realized that the workpieces Z8 are quickly and easily removed and replaced on the fixtureincluding the arm 51a. The metal, such as the lead 6l, in thecylindrical container 60, must be relatively non-soluble in aluminum,and vice Versa. Lead is such a metal.

Also it will be noted that when the fixture 50, modified as shown inFIGURE 8, is removed from a furnace and permitted to cool, the lead 61or other Similar metal in the container 60, solidies and thus is notlost or separated from the fixture, as would bevapt to be the case withmoving clamping parts and the like.

A further development of the lead bath electrical connection isillustrated in FIGURE 10. Here is shown a two-section furnace, similarto that previously discussed and shown in FIGURE 7, similar parts beingidentified with the same reference characters as in FIG- URE 7. Thefurnace in FIGURE l0 is modified, however, to include a molten lead pool70 in the bottom of the pretreating section 39, and a relatively deepwell 71 formed in the partition 36, also containing molten lead. The two`baths 70 and 71 of molten lead are interconnected by abus bar 72 whichmay, if desired, be embedded in the material of which the partition 36is constructed.

The pool 70 forms an electrical connection for long work pieces orfixtures, such as the bar 73, the upper end of the bar being supported,if necessary, in an external frame 74 so as to prevent shorting of thebar 73 against the shroud electrode 37. Electrical interconnectionbetween the molten aluminum i3 and the lead bath 71 may be accomplishedby a hook-shaped fixture such as that indicated at 75, and the latterVelectrode may also serve t'o support a work piece 76 for the coatingoperation. The shank portion 77 of the hook 75 is of sufficient lengthand the lead bath well 7l is of suiiicient depth so It) that the workpiece 76 can be drawn entirely through the molten aluminum 43 `beforethe shank 77 is withdrawn from the lead bath 71.

When it is desired to coat the lower end only of a long bar, such asthat indicated at 73, the same may be first pretreated by inserting itin the pretreating section 39 with the lower end thereof dipping intothe lead bath 7d whereby to perform the electrolytic action in themanner previously described, the circuit being completed by a suitablehook in the well 7i extending into the aluminum 43. The bar 73 maythereafter be removed and dipped in the molten aluminum d3, externalelectrical potential being supplied if desired.

A still further modified form of the two-section furnace of FIGURE 7 isshown in FIGURE 9, the same reference characters being employed toindicate similar parts. A pair of rollers Si? are mounted on the upperedge of the end walls of the furnace 3S, and a similar roller 81immersed in the salt bath is mounted on the lower edge of theintermediate partition 36. Thus a continuous strip or wire 82 may passover the rollers Si) and under the roller Si, and be moved in thedirection of the arrows whereby to be continuously pretreated and coatedby passing first downwardly through the shroud electrode 37, andthereafter upwardly through the electrically positive aluminum bath 43.The wire titi is then preferably oilquenched. If desired, the strip 82may be constructed in the form of an endless chain (not shown) havingintermittent carriers or hooks thereon, whereby individual articles maybe carried continuously down through the pretreating section 39 andupwardly through the aluminum 43.

While the forms of the device and the processes shown and describedherein are fully capable of achieving the objects and providing theadvantages hereinbefore stated, it will be realized that they arecapable of some modification without departure from the spirit of theinvention. For this reason, I do not mean to be limited to the formsshown and described, but rather to the scope of the appended claims.

I claim:

1. The processof coating base metals with a metal selected from thegroup consisting of aluminum and aluminum alloys which comprises:electrolytically pickling said base metal in a molten salt bath whereinsaid base metal is made anodic and said salt bath is made cathodic; `andimmersing said base metal into the coating metal, without atmosphericcontact, said base metal being coated thereby.

2. The process of coating `base metals with a metal selected from thegroup consisting of aluminum and aluminum alloys which comprises:electrolytically pickling said base metal in a molten salt bath whereinsaid hase metal is made anodic and said salt bath is made cathodic;immersing said pickled base metal into said coating metal, withoutintervening atmospheric contact, the base metal being maintainedpositive ywith respect to the salt bath throughout the coating; andsubstantially immediately quenching said coated base metal.

3. The process of coating -a base metal with a coating metal, selectedfrom the group consisting of aluminum and aluminum alloys, whichcomprises: floating a molten bath of said coating metal on a heavierfused salt bath; passing a direct electric current between said basemetal and said salt bath, the direction of the current passage beingsuch as to make said base metal positive with respect to said salt bath;immersing said base metal in said salt bath; moving said base metaldirectly from saidV salt bath, without any atmospheric contact, intosaid molten metal; and thereafter removing said base metal from saidcoating metal whereby a l-ayer of molten metal adheres to said basemetal.

4. The process of coating a base metal with a coating metal, selectedfrom the group consisting of aluminum and aluminum alloys, whichcomprises: floating a molten bath of said coating metal on a portion ofa heavier fused salt bath; passing a direct electric current betweensaid base metal and said salt bath, the direction of the current passagebeing such as to make said base metal positive with respect to said saltbath; immersing said base metal in said salt bath; moving said basemetal directly from said salt bath, without any atmospheric contact,into said molten metal; thereafter removing said base metal from saidcoating metal whereby a layer of molten metal adheres to said basemetal; and immediately quenching said coated base metal.

5. The method of claim 4 wherein said quenching is performed in an oilbath sufficiently large to rapidly lower the temperature of the basemetal and prevent excessive interfacial alloy formation.

6. The method of claim 4 wherein said quenching is performed in ambientair.

7. The method of claim 4 wherein said base metal is selected from thegroup consisting of titanium, vanadium, chromium, manganese, iron,cobalt, nickel and copper.

8. The method of claim 4 wherein said base metal is electricallyinsulated from said coating metal except on its withdrawal into saidcoating metal for the coating of said base metal.

9. The process of coating a base metal selected from the groupconsisting of titanium, vanadium, chromium, manganese, iron, cobalt,nickel and copper, with a coating metal selected from the groupconsisting of aluminum and aluminum alloys, which comprises: iioating amolten bath of said aluminum coating metal on a portion of a heavierfused halide salt bath; immersing said base metal in said salt bath;passing a direct electric current between said base metal and said saltbath, the direction of the current passage being such as to make saidbase metal positive with respect to said salt bath; moving said basemetal directly from said salt bath without any atmospheric contact, intosaid molten metal after said base metal has approximately attained thesalt bath temperature; removing said base metal from said coating metalbath whereby a layer of molten metal adheres to said base metal; andimmediately quenching said coated base metal in `an oil-bath whichremains at a temperature of below about 500 F. throughout the quench.

l0. The process of coating a base metal with a coating metal selectedfrom the group consisting of aluminum and aluminum alloys whichcomprises: floating a molten bath of said coating metal on a heavierfused halide salt bath; passing said base metal into said molten saltbath; maintaining a potential between said base metal and said salt bathwhereby said base metal is made anodic and said salt bath cathodic;commencing withdrawal of said base metal from said salt bath and justprior to its withdrawal reversing the direction of current iiow betweensaid base metal and salt bath whereby said base metal becomes cathodicwith respect to said salt bath; completing the withdrawal of said basemetal from said salt bath; moving said base metal Idirectly into saidcoating metal; thereafter removing said base metal from said coatingmetal bath', and substantially immediately thereafter cooling saidcoated base metal in ambient air whereby a thin base metal-coating metalinterfacial alloy is formed in conjunction with a thickened coatingmetal layer.

l1. The method of claim l wherein said base metal isV selected from thegroup consisting of titanium, vanadium, chromium, manganese, iron,cobalt, nickel and cooper.

l2. The process of coating a base metal with a coating metal selectedfrom the group consisting of aluminum and aluminum alloys, whichcomprises: iioating a molten bath of said aluminum coating metal on aportion of a heavier fused halide salt bath; immersing said base metalin said salt bath; passing a direct electric current between said basemetal and said salt bath, the direction of the current passage beingsuch as to make 4said base metal positive with respect to said saltbath; moving said base metal directly from said salt bath without anyatmospheric contact, into said molten metal after said base metal hasapproximately `attained the salt bath temperature; removing said basemetal from said coating metal bath whereby a layer of molten metaladheres to said base metal; and slowly cooling said coated base metal.

13. The process of coating a continuous base metal with a coating metalselected from the group consisting of aluminum and alumimun alloys,which comprises: floating a molten bath of said aluminum coating metalon `a portion of a heavier fused halide salt bath; immersing said basemetal in said salt bath; passing a direct electric current between saidbase metal and said salt bath for at least -a portion of the travel ofsaid base metal through said salt bath, the direction of the currentpassage being such as to make said base metal positive with respect tosaid salt bath; moving said base metal directly from said salt bathwithout any atmospheric contact, into said molten metal after said basemetal has approximately attained the salt bath temperature; removingsaid base metal from said coating metal bath whereby a layer of moltenmetal adheres to said base metal; and substantially immediately coolingsaid coated base metal.

14. 'Ihe process of coating a base metal with a coating metal, selectedfrom the group consisting of aluminum and aluminum alloys, whichcomprises: iioating a molten bath of said aluminum coating metal on aportion of a heavier fused halide salt bath; passing said base metalinto said molten salt bath; commencing the withdrawal of said base metalfrom said salt bath; maintaining lan electric potential between saidbase metal and said salt bath whereby said base metal is anodic andcurrent iiow is directed from the base metal to said salt bath duringimmersion of said base metal `in said salt bath, but just prior to thecommencement of said withdrawal of said base metal reversing thepolarities whereby said base metal becomes cathodic with respect to saidsalt bath; completing the withdrawal of said base metal from said saltbath; moving said base metal directly into said coating metal;thereafter removing said base metal from said coating metal bath; andsubstantially immediately cooling said coated metal.

15. The process of claim 3 wherein said base metal is maintainedelectrically positive with respect to said salt bath during its passagethrough .said coating metal.

16. The process of claim 3 wherein said base metal ismaintainedelectrically negative with respect to said salt bath during its passagethrough said coating metal.

17. Apparatus for pre-treating and coating metal objects which includes:-a salt bath furnace having a crucible and a bath of fused salt thereinadapted to support a layer of molten metal thereon; a first processelectrode disposed in said salt bath; a second process electrodedisposed adjacent the upper face of said bath to contact a layer ofmolten metal thereon; means for connecting said electrodes to a sourceof direct current whereby to make said second electrode anodic, thedirection of current flow being from said second electrode to said firstelectrode; and means for supporting a work piece immersed in said saltbath and establishing electrical contact between said work piece andsaid molten metal.

18. Apparatus for pre-treating and coating metal objects which includes:a salt bath furnace having a nonconductive crucible and a bath of fusedsalt therein adapted to support a layer of molten metal thereon; a firstprocess electrode affixed to, and within, said Crucible disposed in saidsalt bath; a second process electrode afiXed to, and within, saidCrucible disposed 4adjacent the upper face of said bath to contact alayer of molten metal thereon; means for connecting said electrodes to asource of direct current in a manner to make said second electrodeanodic; and means for supporting a work piece immersed in said salt bathand establishing electrical contact between said work piece and saidmolten metal, whereby current may ow from said second electrode throughsaid work piece and salt bath to said rst electrode.

19. An apparatus according to claim 18 wherein said supporting meansincludes a container portion having a body of molten metal therein, saidcontainer being adapted to receive a conductor member electricallyconnected to said work piece.

20. An apparatus according to claim 18 wherein said supporting meanscomprises a ixture supported for movenient to reverse its position,whereby to place the portion previously in one section into the othersections, and vice versa.

21. An apparatus according to claim 18, in which said Crucible isprovided with `a transverse partition dividing the same into twosections; said first electrode being disposed in one of said sections,and said supporting means being bifurcated and having portions extendinginto each section.

References Cited in the 'Eile of this patent UNITED STATES PATENTS BettsAug. 1, Chance Oct. 3, Rosseau Nov. 9, Dellgren Nov. 1, Moller Apr. 6,Tainton May 18, Clenny May 25, Webster Apr. 19, Spence et al. Ian. 16,Burkhardt Apr. 2,

OTHER REFERENCES ser. No. 369,610, Helling et ai. (APC), May is, 1943.

1. THE PROCESS OF COATING BASE METALS WITH A METAL SELECTED FROM THEGROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES;ELECTROLYTICALLY PICKING SAID BASE METAL IN A MOLTEN SALT BATH WHEREINSAID BASE METAL IS MADE ANODIC AND SAID SALT BATH IS MADE CATHODIC;